EP1592082A1

EP1592082A1 - Contact-free element of transition between a waveguide and a microstrip line

Info

Publication number: EP1592082A1
Application number: EP05103289A
Authority: EP
Inventors: Dominique Lo Hine Tong; Philippe Minard; Corinne Nicolas; Ali Louzir; Julian Thevenard; Jean-Philippe Coupez; Christian Person
Original assignee: Thomson Licensing SAS
Current assignee: THOMSON LICENSING
Priority date: 2004-04-29
Filing date: 2005-04-22
Publication date: 2005-11-02
Anticipated expiration: 2025-04-22
Also published as: MXPA05004426A; US20060097819A1; DE602005022013D1; JP2005318632A; JP4516883B2; EP1592082B1; KR20060045853A; US7746191B2; FR2869725A1; BRPI0501576A; KR101158559B1

Abstract

The present invention relates to an element of transition between a waveguide 10 and a transition line on a substrate 30. The element of transition comprises a securing flange 20 on the substrate 30, the flange 20 being dimensioned so that at least, in the direction microstrip line, the width d of the flange is selected in such a manner as to shift the resonant modes away from the useful band.The invention is used particularly for circuits using SMD techniques at millimeter frequencies.

Description

The present invention relates to an element of transition between a microstrip technology line circuit and a waveguide circuit, more particularly a contact-free transition between a microstrip technology feeding line and a rectangular waveguide realized by using metallized foam based technology.
Radio communication systems that can transmit high bit-rates are currently experiencing strong growth. The systems being developed, particularly the point-to-multipoint systems such as the LMDS (Local Multipoint Distribution System) systems, WLAN (Wireless Local Area Network) wireless systems, operate at increasingly higher frequencies, namely in the order of several tens of Giga-Hertz. These systems are complex but must be realized at increasingly lower costs owing to their consumer orientation. There are now technologies such as LTCC (Low Temperature Cofired Ceramic) or HTCC (High Temperature Cofired Ceramic) technologies that enable devices integrating passive and active functions operating at the above frequencies to be realized at low cost on a planar substrate.
However, some functions are difficult to realize in the millimetric band, particularly filtering functions, because the substrates that must be used in this case do not have the qualities required at the millimetre-waveband level. This type of function must therefore be realized by using conventional structures such as waveguides. Problems then arise with the interconnection of the waveguide device and the printed circuit realized using microstrip technology designed for use by the other functions of the system.
On the other hand, for identical reasons linked mainly with millimeter frequencies, the antennas and their associated elements, such as filters, polarizers or orthomodes, are also realized using waveguide technology. It is therefore necessary to be able to connect the circuits realized using waveguide technology to the planar structures realized using conventional printed circuit technology, this latest technology being suitably adapted for mass-production.
Consequently, many studies have been conducted on the interconnection between a waveguide structure and a planar structure in microstrip technology. Hence, the article of the 33^rd European Microwave Conference at Munich, in 2003, page 1255, entitled "Surface mountable metallized plastic waveguide filter suitable for high volume production" of Muller et al, EADS, describes a waveguide filter capable of being connected to multilayer PCB (Printed Circuit Board) circuits by using the SMD (Surface Mounted Device) technique. In this case, the input and output of the waveguide filter are soldered directly onto footprints realized on the printed circuit. These footprints supply a direct connection to a microstrip line. Hence, the excitation of the waveguide mode is carried out by direct contact between the microstrip access lines and the guide structure. This transition therefore proves complicated to realize and requires stringent manufacturing and positioning tolerances.
A transition between a rectangular waveguide and a microstrip line has also been proposed in French patent 03 00045 filed on 3 January 2003 in the name of THOMSON Licensing S.A. This transition requires modelling the extremity of the waveguide in a particular manner and realizing the microstrip line on a foam substrate extending the foam structure in which the ribbed waveguide is realized. In this case the foam bar forming the waveguide is also used as substrate for the microstrip line. This type of substrate is not always compatible with the realization of passive or active circuits.
In all cases, the embodiments described above are complex and inflexible.
The present invention therefore proposes a new type of contact-free transition between a waveguide structure and a structure realized using microstrip technology. This transition is simple to realize and allows wide manufacturing and assembly tolerances. Moreover, the transition of the present invention is compatible with the SMD mounting technology.
The present invention relates to an element of transition for a contact-free connection between a waveguide circuit and a microstrip technology line realized on a dielectric substrate. The transition element extends the extremity of the waveguide by a flange for securing to the substrate, said substrate featuring a conductive footprint for realizing the connection with the lower surface of the flange. In addition, to realize the adaptation of the transition, a cavity is realized opposite the extremity of the waveguide under the substrate, this cavity presenting specific dimensions.
Preferably, the waveguide circuit and the securing flange are realized in a block of synthetic material such as foam with the external surfaces metallized except for the zone opposite the cavity.
Moreover, the securing flange is preferably integral with the extremity of the waveguide. However, for some embodiments, the securing flange is an independent element being fixed to the extremity of the waveguide.
According to a first embodiment, the securing flange is dimensioned so that, at least in the direction of the microstrip line, the width d of the flange is chosen to shift the resonating modes away from the useful bandwidth, the securing flange being at least perpendicular to the extremity of the waveguide. In this case, the cavity has a depth equal to λ/4 where λ corresponds to the guided wavelength in the waveguide and the microstrip line terminates in a probe.
According to a second embodiment, the securing flange is realized in the extension of the waveguide. In this case, the microstrip line preferably terminates in a capacitive probe and the cavity has a depth between λ/4 and λ/2 where λ corresponds to the guided wavelength in the waveguide. To prevent electrical leakage, the conductive footprint realized on the substrate to enable the connection with the C-shaped flange, the opening between the branches of the C being dimensioned to limit the leakage of electrical fields while preventing short-circuits.
According to a third embodiment, the waveguide is formed by a hollowed out block of dielectric material of which the outer surface is metallized. In this case the C shaped conductive footprint realized on the substrate extends in the direction of the guide in such a manner as to form the lower part of the waveguide. The footprint must preferably comprise a first metallized zone to which the waveguide is welded and a second metallized zone inside the first and forming a cover for the waveguide.
Other characteristics and advantages of the present invention will emerge upon reading the description of diverse embodiments, this reading being made with reference to the figures attached in the appendix, in which:
Figure 1 is an exploded perspective view of a first embodiment of an element of transition between a waveguide circuit and a microstrip technology line in accordance with the present invention.
Figure 2a and figure 2b are respectively a top view and bottom view of the substrate comprising the microstrip technology line used in the first embodiment.
Figure 3 is a perspective view of the transition element integrated with the waveguide.
Figure 4a and figure 4b are curves giving, for the embodiment of figure 1, the adaptation as a function of the frequency for a dimension d of the flange in the direction of the microstrip line, such as d = 4 mm and d = 2.3 mm respectively.
Figure 5 is an exploded perspective view of an element between a microstrip line and a waveguide bent at 90°, according to a variant of the first embodiment.
Figure 6 gives the impedance matching and transmission loss curves as a function of the frequency for the embodiment of figure 5.
Figure 7 represents an exploded perspective view of another variant of the first embodiment, for a waveguide with two 90° bends.
Figure 8 gives the impedance matching and transmission loss curves as a function of the frequency for the embodiment of figure 7.
Figure 9 is a curve showing the variations in the resonant frequency as a function of the dimension d, enabling the limit values of d to be determined.
Figure 10 is an exploded perspective view of a second embodiment of an element of transition between a waveguide circuit and a microstrip technology line in accordance with the present invention,
Figure 11a and 11b are respectively a top view and bottom view of the substrate comprising the microstrip technology line used in the second embodiment,
Figure 12 shows the insertion and return loss curves simulated for a transition: waveguide circuit and microstrip line according to figure 10,
Figure 13 is a magnified bottom view showing the conductive footprint and the microstrip line on the substrate for an embodiment of figure 10,
Figure 14 is a curve giving the insertion losses as a function of the opening width of the footprint for the embodiment of figure 10 at 30 GHz,
Figures 15, 16, 17 show the return loss curves for different footprint dimensions,
Figures 18a and 18b respectively show an exploded perspective view of a variant of the embodiment of figure 10 for a waveguide circuit comprising an SMD filter and the impedance matching and return loss curves simulated for this variant and,
Figures 19a and 18b respectively show an exploded perspective view of another variant of the embodiment of figure 10 for a waveguide circuit comprising an SMD pseudo-elliptic filter and the impedance matching and return loss curves simulated for this variant.
Figure 20 is an exploded perspective view of a second embodiment of an element of transition between a waveguide circuit and a microstrip technology line in accordance with the present invention,
Figures 21 a and 11 b are respectively a bottom view and top view of the substrate comprising the microstrip technology line used in the third embodiment, and
Figure 22 shows the insertion and return loss curves simulated for a transition according to figure 20:
A first description with reference to figures 1 to 4 will be made for a first embodiment of an element of transition between a waveguide circuit and a microstrip line realized on a dielectric substrate.
As shown diagrammatically in figure 1, which relates to an exploded view of the element of transition, the reference 10 diagrammatically shows a rectangular waveguide. This waveguide is preferable realized in a synthetic material, more particularly in foam with a permittivity noticeably similar to that of air. The rectangular block of foam is metallized, as referenced by 11, on all the external surfaces so as to realize a microwave waveguide.
As shown particularly in figures 1 and 3, a flange 20, which presents a noticeable "C" shape, is realized at one end of the guide 10, preferably at the same time as the foam technology waveguide. This flange 20 surrounds the rectangular extremity of the guide 10 on its two smaller sides 21 and on one of its large sides while the other large side has an opening 22 positioned in such a manner as to prevent any short circuit with the microstrip line 31 realized on a dielectric substrate 30, as will be explained subsequently.
As shown more clearly in figure 3, the assembly formed by the rectangular waveguide and the element of transition constituted by the flange is metallized in 11 and 23. However, the extremity corresponding to the output of the guide forming a rectangular zone together with the zone that is vertically at the level of the break in the flange 20 are non-metallized as shown by 24.
This flange 20 constituted by a partly metallized foam structure forms a hyperfrequency cavity that can disturb and degrade the transition performances. To prevent this problem and in accordance with the present invention, the flange 20 was dimensioned specifically to obtain a reliable electric contact with the substrate carrying the microstrip technology circuits as will be explained hereafter, while ensuring good mechanical support for the assembly and by eliminating the resonating modes.
Hence, the part of the flange 20 opposite the non-metallized part 22, which corresponds to the part opposite the microstrip line, is dimensioned so as to shift the resonance frequency of the flange outside the useful band. The thickness of the flange being selected according to the mechanical strength required, the dimension d of this part of the flange will be selected such that the resonant frequency generated is outside the useful band. Moreover, as shown in figure 1, the microstrip technology circuits are realized on a dielectric substrate 30. In a more specific manner, as shown in figure 2, the dielectric substrate 30 comprises a metal layer 30a forming a ground plane on its lower face with a rectangular non-metallized zone 30b corresponding to the rectangular output of the waveguide 10 and next to a cavity 41 realized in the box or base 40 supporting the substrate 30, as will be explained hereafter.
The upper face of the substrate shown in figure 2a comprises a microstrip technology line 31a that is extended by an impedance matching line 31 b using microstrip technology and a connection element or probe 31 c for recovering the energy emitted by the waveguide 10. This element normally being known under the English term "Probe".
To enable the connection between the waveguide output and the probe 31c, a footprint 30c of the lower face of the flange 20 was realized in a conductive material on the upper face of the substrate 30. As clearly shown in figure 2a, the part of the footprint being found in the extension of the probe 31c has a width d corresponding to the width d of the part of the flange 20 shown in figure 1.
The metallized zone 30c is used to receive the equivalent surface of the flange which is connected by welding, more particularly by soldering, and this zone is connected electrically to the ground plan below 30a by metal holes not shown.
Moreover, as shown in figure 1, the dielectric substrate receiving the microstrip technology circuits is mounted on a metal base or metal box 40 featuring a cavity 41 in the part facing the waveguide. This cavity has an opening equal to that of the rectangular waveguide and a depth noticeably equal to a quarter of the wavelength guided in the waveguide, this is to provide impedance matching for the transition.
For the present invention, it appears that only the width of the part of the flange of the element of transition found in the same direction as the microstrip technology line is of importance with respect to resonance phenomena. Indeed, for a rectangular waveguide as shown in figure 1, the fundamental mode TE10 is excited and the electric field is maximum in the axis of the access line and quasi-null laterally on the small sides of the guide. Hence, the cavities located on either side of the microstrip line and formed by the lateral parts of the flange, have little effect on the performances and the dimensions of these parts of the flange are selected only to provide mechanical rigidity for the assembly. On the contrary, with respect to the rear flange part, it is excited by the feeding line, which creates a resonant frequency depending on the dimensions of this part, this frequency being able to fall within the useful band. The width d is therefore chosen to shift this frequency from the useful band, the height being chosen according to mechanical constraints.
To validate the concept described above, an element of transition associated with a planar structure and a rectangular waveguide of the type of that in figure 1 was simulated electromagnetically in 3D by using simulation software known under the name "Ansoft/HFSS" that implements a finite elements method. In this case, a waveguide of name WR28 having a guide cross-section of 3.556 mm x 7.112 mm is extended by a flange such as shown in figure 1. The flange, which has a thickness of 1.5 mm, a width on the small sides of 2 mm and a width equal to 4 mm or 2.3 mm, was mounted as described above on a low-cost microwave substrate of thickness 0.2 mm, known commercially under the name of RO4003 on which a microstrip line was realized.
Moreover, the waveguide is realized by metallizing a foam material known under the commercial name "Rohacell/HF71" which presents a very low dielectric constant and low dielectric loss where, in particular, εr = 1.09, tg. δ = 0.001, up to 60 GHz. The results of the simulations are given in figure 4a, where d = 4 mm, and in figure 4b, where d = 2.3 mm.
It is observed that, for d = 4 mm, an excellent impedance matching of around 18 dB is obtained over a frequency band of 27 to 32 GHz, whereas, for d = 2.3 mm, a disastrous resonance is observed at around 29 GHz.
In figure 5, an embodiment variation of the present invention was shown. In this case, the waveguide 100 is a guide bent at 90°, as shown by the reference 101, comprising a flange 102 at its extremity, the assembly being realized using foam technology, namely by milling a foam block and covering it with a metal layer, as described above. The flange 102 is a flange of the same type as the flange shown in figure 1. This flange has a "C" shape and features an opening 103 in the part that must face the microstrip technology feeding line to be coupled to the waveguide.
As shown in figure 5, a substrate 110 of the same type as the substrate 30 of figures 1 and 2, features a microstrip technology feeding line 111 and a conductive footprint 112 for securing the flange 102. This footprint 112 presents, in the part opposite the feeding line 111, a dimension d with a value determined as mentioned above in a manner that shifts the resonant frequency of this part out of the useful band.
In an identical manner to the embodiment of figure 1, this substrate is mounted on a metal base or metal box with a cavity 121, the height of which is equal to λ/4, λ being the guided wavelength in the waveguide.
A system of this type was simulated by using the same software as above, with the same types of materials for the substrate and the guide. The dimensions of the bend 101 were optimised for an application at around 30 GHz. The curve for impedance matching as a function of the frequency is shown in figure 6. It shows impedance matching of more than 20 dB on 1 GHz of bandwidth around 30 GHz.
In figure 7, another embodiment variation was shown with a double waveguide/planar substrate transition, more particularly a straight waveguide 200 realized using foam technology extending at each extremity by a 90° bend 201a, 201 b, each curve extremity extending by a flange 202a, 202b such as the one described with reference to figure 5. This flange is used to connect the waveguide 200 to input circuits and output circuits realized in microstrip technology on a planar substrate 210, in a microwave dielectric material. At the level of the transition of each waveguide extremity with the microstrip lines on the substrate, footprints 211a, 211b of the same type as the footprint 112 in figure 5 were realized. These footprints surround a non-metallized part 213a, 213b in which arrives the extremity (or probe) of a microstrip line 212a, 212b being used to supply the circuits realized using planar technology. The substrate 210 is mounted on a metal base or metal box 220, featuring, as for figure 5, cavities 221 a, 221 b, opposite the extremities 201a, 201 b of the waveguide 200. The cavities are dimensioned as in the embodiment of figure 1.
A structure of this type was simulated as mentioned above and the results of the simulation in terms of impedance matching are shown in figure 8.
In this case, the level of loss is close to the loss obtained for a single transition at 30 GHz and the insertion loss simulated is less than 1.5 dB for a waveguide length of 42 mm.
As mentioned above, the dimension d is selected so that the cavity formed by the part of the flange opposite the part corresponding to the microstrip line resonates at a frequency that is outside the frequency of the useful band. To accomplish this, the resonant frequency of this part depends not only on the value d but also the height and width of this part of the flange. These last two dimensions are selected so that the flange is mechanically rigid. Therefore, d is a value inversely proportional to the frequency for a chosen height and base width. The curve of figure 9 gives the variation in the resonant frequency as a function of the width d of the flange. For example, for a system operating in the 27 to 29 GHz bandwidth, the value of d must be greatly superior to 2.5 mm so that the resonant frequency is displaced far from the useful bandwidth.
A description will now be given, with reference to figures 10 to 17, of another embodiment of an element of transition in accordance with the present invention. In this case, the waveguide circuit 50 comprises a rectangular waveguide 51, the extremity of which is extended by a flange 52 for securing on a substrate 60 featuring planar technology circuits, particularly microstrip.
In this embodiment, the lower plane 52a of the flange 52 extends the lower part 51 a of the rectangular guide in such a manner that the entire waveguide rests on the substrate 60. Moreover, the extremity of the rectangular guide terminates by a bevelled part 53. As for the first embodiment, the rectangular waveguide 50 is realized in a solid block of synthetic foam, which can be of the same type as the one used in the realization of figure 1. The outer surface of the guide and the flange is metallized, with the exception of a zone 54, rectangular in the embodiment shown and which is located above the impedance matching cavity 71 subsequently described in more detail and a zone 55 situated vertically at the interface between the microstrip technology line and the foam block to prevent any short-circuit.
To realize a contact-free connection with planar technology circuits, more particularly microstrip technology, the substrate 60 in dielectric material comprises, as shown in figures 1, 2a and 2b, a lower ground plane 60a featuring a non-metallized zone 60b in the part located opposite the cavity 71.
On the upper plane 60c of the substrate, an access line 60 terminating in a probe 60e, which, in the present case was dimensioned to be capacitive, are realized in microstrip technology.
Moreover, to realize the attachment of the waveguide 50 to the substrate 60, the probe 60e is surrounded by a conductive footprint 60f with a form that corresponds to the lower surface of the flange 52. The attachment of the flange to the footprint is made by welding, particularly by soldering or any other equivalent means. The shape of the footprint will be explained in more detail hereafter. Moreover, the footprint 60f is electrically connected to the ground plane 60a by metallized holes not shown.
The substrate 60 is, moreover, mounted on a metal base or a metal unit 70 which, for the present invention, comprises at the level of the transition a cavity 71 molded or milled in the base 70. The cavity 71 preferably has a cross-section equal to that of the rectangular waveguide and a depth of between λ/4 and λ/2, where λ represents the guided wavelength in the waveguide. The exact dimension of the depth is chosen so as to optimise the response of the element of transition.
In this embodiment, the dimensioning of the flange is realized to facilitate the correct offset of the waveguide on the substrate but also to provide a reliable electrical contact with the printed circuit to provide earth bonding for the entire assembly while avoiding power leakage at the level of the transition. Now, the flange comprises a hyperfrequency cavity that can interfere with and degrade the performances of the transition. It must therefore be dimensioned correctly.
In this case, the TE10 mode is excited. Therefore, the configuration of the electric field is maximum in the axis of the access line and almost null laterally on the small side of the guide.
Therefore, the flange parts forming cavities located on either side of the access line have few spurious effects on the performances of the system. However, the dimensioning of the opening 55 in the flange 52, essential to the input of the microstrip line 60d, is critical. It is necessary to offer an adequate space to prevent disturbances linked to the coupling between the microstrip access line and the metallized zones of the flange. Conversely, an opening that is too large will directly contribute to the significant increase in leaks, this opening being located in a high concentration zone of the electric field.
The embodiment described below was simulated by using a method identical to the one described for the embodiment of figure 1. Hence, for an element of transition between a microstrip line realized on a low cost substrate made of a dielectric material of the name ROGERS RO4003 of thickness 0.2 mm and a waveguide as shown in figure 10 realized with low loss material (such as a foam known under the commercial name ROHACELL HF71) of standard cross-section WR28: 3.556 mm x 7.112 mm and height 1 mm; the results of the simulation with a dimensioning of the guide designed to operate around 30 GHz are shown in figure 12.
In this case, the following is obtained:

An impedance matching of more than 20dB in a very large bandwidth ranging from 22.2 to 30.8 GHz.
An impedance matching of more than 25dB from 28.9 to 30.1 GHz.
Fairly low insertion losses in the order of 0.25 dB.

The influence of dimensions given for the flange 52 on the optimization of the transition will now be described with reference to figures 13 to 17. Figure 13 diagrammatically showed a top view of the element of transition when the waveguide is mounted on the substrate. In this case, the flange 52 comprises two projecting lateral cavities 52b with respect to the lateral walls of the guide 51 itself. These two cavities extend by a perpendicular cavity 52a featuring an opening 52c in its middle, corresponding to the passage of the microstrip line. In this embodiment, as mentioned above, the dimensions of the opening 52c have an impact on the electrical performances of the transition such as insertion losses (S21) and return losses (S11).
Hence, as shown in figure 14, which gives the insertion losses S21 as function of the width of the opening 52a, 3 distinct zones can be noted:
For an opening less than 0.8 mm, the losses are high, this reflecting the phenomenon of coupling between the line and the metallized walls of the guide.
For an opening varying from 0.8 to 2 mm, we observe a range of optimum values for which the transmission losses are minimum and in the order of -0.25 dB.
For an opening greater the 2 mm, the losses begin to increase, thus resulting in an increase of field leakage.
Moreover, figure 15 shows the return losses as a function of the width d of the openings found for each of the 3 previous zones. The following is therefore observed:
For an opening less than 0.8 mm, the return loss response of the structure is totally disturbed. The presence, too close, of the extremity of the cavity introduced a notable mismatching.
For an opening varying from 0.8 to 2 mm, the impedance matching is optimum and covers the working bandwidth.
For an opening greater than 2 mm, the beginning of a rise in levels that is related to the leakage by the opening that is too large.
Figures 16 and 17 show the influence of the widths a and b of the cavities 52a, 52b forming the flange on the performances of the transition.

Concerning the cavity a, figure 16 shows that the width of this cavity has only a small effect on the return loss response of the transition, the losses always remain below -15 dB, in a wide frequency band, and this for widths varying widely from 0.2 to 1.5 mm.
Concerning the width of the cavity b, figure 17 shows that it disturbs the transition performances even less, since by doubling its value from 1 mm to 2 mm, the return losses always remain less than -17dB in a very wide range of frequency bands.

Figures 18 and 19 diagrammatically show two embodiment variants of the waveguide circuit used with an element of transition of the type described with reference to figure 10.
For figure 18, the waveguide 500 is an iris waveguide filter of the order of 3 showing a Chebyshev type response. The guide 500 is connected to planar technology circuits by using an element of transition as described above. Hence, figure 18a diagrammatically shows the substrate 501 featuring connection footprints and access lines and the base 502 featuring a cavity opposite the output of the filter 500.
The performances associated with this embodiment are shown in figure 18b. The following can be noted:
Low insertion losses in the order of 1.2dB, for a frequency range of 900 MHz around 30 GHz.
Return losses lower than -23dB on this same frequency range.
Figure 19 is similar to figure 18 and shows a waveguide 600 containing a pseudo-elliptic filter comprising 2 stubs placed at each input of the guide. The purpose of this device is to create 2 transmission zeros locally outside of the bandpass thus increasing the selectivity of the filter. This surface mounted filter 600 on a substrate 601 RO4003 and a base 602 featuring a cavity and excited by 2 microstrip lines was fully simulated in 3D. Figure 18b shows the performances obtained:
Insertion losses in the order of 1.2 dB in a pass band of 1 GHz around 30 GHz.
Return losses less than -30dB at the [29.5-30.0] GHz bandwidth.
Attenuation of more than 60dB at 28.55 GHz, the frequency corresponding to a spurious frequency to reject.
A description will now be given, with reference to figures 20 to 22, of another embodiment of an element of transition in accordance with the present invention. In this case, the waveguide circuit 80 comprises a rectangular waveguide 81 for which the extremity extends by an element 82 forming the securing flange. In this embodiment, the waveguide is formed by a block of dielectric material that can be a synthetic foam of permittivity equivalent to that of air. The block was hollowed out to form a cavity 83 and the outer surface of the block is fully metallized Moreover, the flange 82 has a slot 84 whose role will be explained hereafter. In the embodiment, the lower plane of the flange 82 extends the lower hollowed out part of the rectangular guide 81 such that the waveguide rests on the substrate 90 receiving the planar technology circuits, particularly microstrip.
As shown in figures 20 and 21, the substrate 90 in microwave dielectric material comprises a foam plane marked 94 in figure 21a, this ground plane featuring a non-metallized area 95 in the part that is located opposite the waveguide output at the level of the transition. Moreover, in this embodiment, the upper plane of the substrate 90 comprises a first metallized zone 93b being used to offset the waveguide 80.
This zone 93b is connected electrically to the ground plane 94 by metallized holes not shown. Moreover, the substrate 90 comprises a second metallized zone 93a placed within the zone 93b and which extends under the entire opening of the waveguide 80 so as to form a cover closing the opening 83 of the waveguide.
The upper face of the substrate 90 also comprises a non-metallized zone 96 corresponding to the zone 95. This zone 96 receives the extremity 92 or "probe" of a feeding line 91 realized in printed circuit technology, particularly microstrip. This line crosses a non-metallized zone in the zone 93a which corresponds to the gap 84 in the flange 82.
The assembly is mounted on a metal base or metal box 72 which, for the present invention, comprises a cavity 73 at the level of the transition molded or milled in the base. The cavity has a cross-section noticeably equal to that of the waveguide extremity, namely, corresponding to the non-metallized zone 95 and a depth of between λ/4 and λ/2, where λ represents the guided wavelength in the waveguide.
The embodiment described above was simulated by using a method identical to the one described for the previous embodiments. Hence, the substrate is constituted by a dielectric material known under the name of ROGERS RO4003 of thickness 0.2 mm. The waveguide is realized in a block of dielectric material that was milled in such a manner that the inner cross-section of the waveguide is equivalent to the standard WR28: 3.556 mm x 7.112 mm and presents a thickness of 2 mm. The guide was metallized with conductive materials such as tin, copper, etc. The system was designed to operate at 30 GHz.
In this case, as shown in figure 22 which concerns a single microstrip line/waveguide transition, the following is obtained:

an impedance matching of more than 15 dB in a very large bandwidth ranging from 26 GHz and 36 GHz,
fairly low insertion losses in the order of 0.4 dB in this frequency band.

It is evident to those in the art that the waveguide 80 described above can be modified to realize an iris waveguide filter featuring a Chebyshef type response of the type of the one shown in figure 18 or a pseudo-elliptical filter with 2 stubs placed at each input of the guide of the type shown in figure 19.
It is evident to those in the art that many modifications can be made to the embodiments described above. In particular, one can envisage obtaining an independent element of transition for some embodiments into which the extremity of the waveguide is inserted. The important factor is to realize a contact-free transition that shows no spurious resonance modes.

Claims

Element of transition for a contact-free connection between a waveguide circuit (10, 100, 50, 80) and a microstrip technology line (31, 111, 60d) realized on a dielectric substrate (30, 110, 60, 90), characterised in that the element of transition extends the extremity of the waveguide by a flange (20, 102, 52, 82) for attachment to the substrate, said substrate featuring a conductive footprint (32, 102, 60f, 93) for making the connection to the lower surface of the flange, and in that a cavity (41, 121, 71, 73) dimensioned to realize impedance matching with the waveguide circuit is realized opposite the extremity of the waveguide under the substrate.
Element of transition according to claim 1, characterized in that the waveguide circuit and the securing flange are realized in a block of synthetic material such as a foam with the external surfaces metallized except for the zone opposite the cavity.
Element of transition according to one of claims 1 or 2, characterized in that the securing flange is integral with the extremity of the waveguide.
Element of transition according to one of claims 1 or 2, characterized in that the securing flange is a separate element that fixes onto the extremity of the waveguide.
Element of transition according to one of claims 3 or 4, characterized in that the securing flange is dimensioned so that, at least in the direction of the microstrip line, the width d of the flange is chosen to shift the resonating modes away from the useful band, the securing flange being at least perpendicular to the extremity of the waveguide.
Element of transition according to one of claims 3 to 5, characterized in that the cavity has a depth equal to λ/4 where λ corresponds to the guided wavelength in the waveguide.
Element of transition according to one of claims 3 to 6, characterized in that the microstrip line terminates in a probe.
Element of transition according to claim 3, characterized in that the securing flange is realized in the extension of the waveguide.
Element of transition according claim 8, characterized in that the cavity has a depth between λ/4 and λ/2 where λ corresponds to the guided wavelength in the waveguide.
Element of transition according to one of claims 8 or 9, characterized in that the microstrip line terminates in a probe.
Element of transition according to one of claims 8 to 10, characterized in that the conductive footprint has a C shape, the opening between the branches of the C being dimensioned to limit the leakage of electrical fields while preventing short circuits.
Element of transition according to one of claims 1 or 3, characterized in that the waveguide is formed by a hollowed out block of dielectric of which the outer surface is metallized.
Element of transition according to claim 12, characterized in that the conductive footprint extend under the hollowed out part of the waveguide so as to form a cover.
Element of transition according to one of claims 12 or 13, characterized in that the conductive footprint realized on the substrate comprises a first metallized zone to which the waveguide is fixed and a second metallized zone inside the first zone, this zone forming a cover for the waveguide.